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    SubjectRe: mmap'ed memory in core files ?
    Hello Hidehiro,

    On Thu, Jul 03, 2008 at 12:51:33PM +0900, Hidehiro Kawai wrote:
    > Hi,
    >
    > Michael Kerrisk wrote:
    >
    > > [CC+= hidehiro.kawai.ez@hitachi.com]
    > >
    > > On Wed, Jul 2, 2008 at 12:50 PM, Philippe De Muyter <phdm@macqel.be> wrote:
    > >
    > >>Hi Michael,
    > >>
    > >>On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
    > >>
    > >>>On 7/1/08, Philippe De Muyter <phdm@macqel.be> wrote:
    > >>>
    > >>>>Hello everybody,
    > >>>>
    > >>>> I develop video acquisition software using the video1394 interface.
    > >>>> The images grabbed by the camera and iee1394 bus are kept in kernel
    > >>>> memory and made available to the user program through a mmap call done
    > >>>> in the libdc1394 library :
    > >>>>
    > >>>> dma_ring_buffer= mmap(0, vmmap.nb_buffers * vmmap.buf_size,
    > >>>> PROT_READ|PROT_WRITE,MAP_SHARED, craw->capture.dma_fd, 0);
    > >>>>
    > >>>> Sometimes, my program crashes and produces a core file :) It seems to
    > >>>> me that the core file does not contain the mmap'ed memory and hence
    > >>>> I cannot replay my program with the same image for debugging purpose.
    > >>>>
    > >>>> Is it possible to configure the kernel through /proc, or through the mmap
    > >>>> system call to have that mmapped segment in the core file, or do I need
    > >>>> to modify the kernel itself to obtain the behaviour I want ? If I
    > >>>> need to modify the kernel, can some kind soul provide me some pointers ?
    > >>>
    > >>>
    > >>>Have a look at the section "Controlling which mappings are written to
    > >>>the core dump" in a recent core.5 man page:
    > >>>http://www.kernel.org/doc/man-pages/online/pages/man5/core.5.html
    > >>
    > >>thanks for the info. I didn't know about /proc/PID/coredump_filter.
    > >>
    > >>that part was promising :
    > >>
    > >> bit 2 Dump file-backed private mappings.
    > >> bit 3 Dump file-backed shared mappings.
    > >>
    > >> The default value of coredump_filter is 0x3; this reflects traditional
    > >> Linux behavior and means that only anonymous memory segments are dumped.
    > >>
    > >>Unfortunately, the part that applies to me (I have tested it) is the next one :
    > >>
    > >> Memory-mapped I/O pages such as frame buffer are never dumped, [...],
    > >> regardless of the coredump_filter value.
    > >>
    > >>Is that a design decision, or a mere finding of the way it is implemented
    > >>now ?
    >
    > MMIO pages have been not dumped since a long time ago, and I didn't target
    > them for the coredump_filter feature because I thought it was generally
    > danger to read MMIO pages. As for frame buffer we would be able to
    > safely access to it, but there is no way to tell it from other MMIO
    > mappings, AFAIK.
    >
    > >>So, back to my original question :
    > >>
    > >>Can some kind soul provide me some pointers to the way I should modify
    > >>the kernel to make the inclusion of the video1394 mmapped segment in
    > >>core files possible ?
    > >
    > >
    > > Perhaps Hidehiro, who wrote the coredump_filter feature, can provide insight.
    >
    > The following patch may help. To dump MMIO pages, set bit 5 of
    > coredump_filter.
    >
    > $ echo 0x23 > /proc/<PID>/coredump_filter
    >
    >
    > Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
    > ---
    > This patch is not intended to be merged to the upstream kernel
    > because the safeness of reading VM_IO mappings has not been
    > proved.
    >
    > Index: linux-2.6.26-rc5-mm3/fs/binfmt_elf.c
    > ===================================================================
    > --- linux-2.6.26-rc5-mm3.orig/fs/binfmt_elf.c
    > +++ linux-2.6.26-rc5-mm3/fs/binfmt_elf.c
    > @@ -1141,11 +1141,18 @@ static unsigned long vma_dump_size(struc
    > if (vma->vm_flags & VM_ALWAYSDUMP)
    > goto whole;
    >
    > - /* Do not dump I/O mapped devices or special mappings */
    > - if (vma->vm_flags & (VM_IO | VM_RESERVED))
    > +#define FILTER(type) (mm_flags & (1UL << MMF_DUMP_##type))
    > +
    > + /* By default, do not dump memory mapped I/O mappings */
    > + if (vma->vm_flags & VM_IO) {
    > + if (FILTER(MMIO))
    > + goto whole;
    > return 0;
    > + }
    >
    > -#define FILTER(type) (mm_flags & (1UL << MMF_DUMP_##type))
    > + /* Do not dump special mappings */
    > + if (vma->vm_flags & VM_RESERVED)
    > + return 0;
    >
    > /* By default, dump shared memory if mapped from an anonymous file. */
    > if (vma->vm_flags & VM_SHARED) {
    > Index: linux-2.6.26-rc5-mm3/include/linux/sched.h
    > ===================================================================
    > --- linux-2.6.26-rc5-mm3.orig/include/linux/sched.h
    > +++ linux-2.6.26-rc5-mm3/include/linux/sched.h
    > @@ -403,8 +403,9 @@ extern int get_dumpable(struct mm_struct
    > #define MMF_DUMP_MAPPED_PRIVATE 4
    > #define MMF_DUMP_MAPPED_SHARED 5
    > #define MMF_DUMP_ELF_HEADERS 6
    > +#define MMF_DUMP_MMIO 7
    > #define MMF_DUMP_FILTER_SHIFT MMF_DUMPABLE_BITS
    > -#define MMF_DUMP_FILTER_BITS 5
    > +#define MMF_DUMP_FILTER_BITS 6
    > #define MMF_DUMP_FILTER_MASK \
    > (((1 << MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
    > #define MMF_DUMP_FILTER_DEFAULT \
    >
    >
    >
    > Regards,

    Thanks for your patch, but it will not help here. Before applying it blindly
    I asked myself if the mmapped region was tagged VM_IO, because it is actually
    a simple ram zone, not an I/O zone, and the answer is it is not a VM_IO zone.
    Details :

    drivers/ieee1394/video1394.c:
    static int video1394_mmap(struct file *file, struct vm_area_struct *vma)
    {
    [...]
    return dma_region_mmap(&ctx->current_ctx->dma, file, vma);
    }

    drivers/ieee1394/dma.c:
    int dma_region_mmap(struct dma_region *dma, struct file *file,
    struct vm_area_struct *vma)
    {
    [...]
    vma->vm_ops = &dma_region_vm_ops;
    vma->vm_private_data = dma;
    vma->vm_file = file;
    vma->vm_flags |= VM_RESERVED;

    return 0;
    }

    So, actually the zone I would like to get dumped in the core file is tagged
    VM_RESERVED.

    I see the following ways to solve my problem :
    - do not tag the zone as VM_RESERVED in ieee1394::dma_region_mmap
    - tag the zone as VM_ALWAYSDUMP in ieee1394::dma_region_mmap
    - add a bit in coredump_filter to dump the VM_RESERVED zones.

    As I don't know the real meaning of VM_RESERVED, I do not know which choice
    is the best one for the official kernel tree, but locally I'll go for
    adding VM_ALWAYSDUMP in ieee1394::dma_region_mmap.

    Philippe


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